1. Field of the Invention
The present invention relates to a magnetic recording medium and a method for manufacturing the same. In particular, the present invention relates to a high recording density magnetic recording medium including a thin magnetic layer, wherein the dilution stability during preparation of magnetic paint is improved.
2. Description of the Related Art
In general, magnetic paint for a magnetic recording medium is manufactured through the steps in which, for example, a magnetic paint composition composed of a magnetic powder, a binder resin, an organic solvent, and other necessary components is supplied to a medium dispersion type mill or the like including a mixing vessel filled in with dispersion media, e.g., glass beads, and is forcedly agitated together with the dispersion media by an agitation device built in the mixing vessel.
In recent years, magnetic layers have been made thinner due to requirements for further increase in the recording density. Accompanying this, the paint concentration of the magnetic paint tends to be lowered. In a commonly used technique for preparing such low concentration paint, dispersion of the paint is performed at a somewhat high concentration, the resulting paint is diluted to attain a final paint concentration, and dispersion is further performed.
With respect to technologies for improving the manufacturing process of magnetic paint used for such a thin magnetic layer, for example, Japanese Unexamined Patent Application Publication No. 2002-367159 discloses a method for manufacturing a magnetic recording medium, in which a predetermined organic solvent is used for the paint to form a magnetic layer and, thereby, a thin magnetic layer can be formed without occurrence of any inconvenience, e.g., reduction in the degree of orientation. Japanese Unexamined Patent Application Publication No. 2001-351224 discloses that the stability of paint is improved by using a phosphoric acid ester based compound and/or phenylphosphonic acid as a dispersing agent in a non-magnetic layer.
As described above, in the preparation of the magnetic paint, a step of diluting the high concentration paint to the low concentration paint is performed. However, as the final concentration is lowered with decrease in the thickness of the magnetic layer, when the high concentration paint after dispersion is diluted to have an aimed low concentration, that is, about 2.0 percent by weight or less in terms of the concentration of the binder resin (=resin/(resin+solvent)) (hereafter may be referred to as “La concentration”), a problem occurs in that the gloss of paint after the dilution is deteriorated compared with the gloss of the paint before the dilution. This deterioration of the dilution stability of the paint deteriorates the surface roughness after application of the magnetic layer and processing, and causes reduction in the output. Therefore, it is required to overcome these problems.